1. Field of the Invention
The present invention relates to an absolute position measuring apparatus. Specifically, the present invention relates to an absolute position measuring apparatus such as a micrometer head, a micrometer, and a Holtest for measuring an absolute position of a spindle that rotatingly moves in an axial direction.
2. Description of Related Art
Conventionally, in a small-sized measuring apparatus for measuring a length, a size or the like such as a micrometer and a micrometer head, an object is measured by detecting a displacement of a movable member relative to a fixed member (for instance, see Document 1: JP-A-2003-207307 (FIGS. 1 and 4)).
An arrangement of a small-sized measuring apparatus disclosed in Document 1 to detect the displacement of the movable member relative to the fixed member will be described with reference to the attached drawings.
FIG. 6 illustrates a conventional micrometer head 101 as the small-sized measuring apparatus disclosed in Document 1.
The micrometer head 101 includes: a main body 102 having a through-hole 121; a spindle 103 that is inserted into the through-hole 121 and advanceable and retractable along the through-hole 121; a phase signal transmitter 104 that transmits a phase signal in accordance with an axial movement of the spindle 103; an arithmetic processor 105 that calculates an absolute position of the spindle 103 on the basis of the phase signal; and a display 106 that displays the calculated absolute position of the spindle 103.
An arrangement of the spindle 103 that is advanced and retracted relative to the main body 102 will be described below.
An internal thread 122 is provided on an inner circumference of the through-hole 121 of the main body 102. The spindle 103 is provided with a feed screw 131 screwed with the internal thread 122 of the main body 102 and a knob 132 on an end of the spindle 103. When the spindle 103 is rotated by the knob 132, the spindle 103 is advanced and retracted in an axial direction relative to the main body 102 due to the screw-engagement between the internal thread 122 and the feed screw 131.
The phase signal transmitter 104 includes two rotary encoders and transmits two phase signals that are different in cycle according to the axial movement of the spindle 103. These two rotary encoders include a stator 141 that is fixed to the main body 102 and two rotors 142 and 145 that are provided on both sides of the stator 141 so as to interpose the stator 141 therebetween.
The stator 141 and the rotors 142 and 145 will be described below in greater detail.
The stator 141 includes a through-hole 148 in the axial direction of the spindle 103 into which the spindle 103 is inserted. Two key grooves 133 and 134 are provided on an outer circumference of the spindle 103. The first key groove 133 is linearly provided along the axis of the spindle 103 and the second key groove 134 is provided spirally around the axis of the spindle 103. A first rotary cylinder 143 having a first key 144 engaged with the first key groove 133 on an inner circumference thereof and a second rotary cylinder 146 having a second key 147 engaged with the second key groove 134 on an inner circumference thereof are provided on an outer circumference of the spindle 103. The rotary cylinders 143 and 146 are provided on both sides of the stator 141 along the axis of the spindle 103.
The rotors 142 and 145 each having openings 149 and 150 respectively, through which the spindle 103 passes in the axial direction of the spindle 103, are provided on the rotary cylinders 143 and 146. In other words, the first rotor 142 is fitted to an outer circumference of the first rotary cylinder 143 and the second rotor 145 is fitted to an outer circumference of the second rotary cylinder 146. The rotors 142 and 145 respectively face each surface of the stator 141.
In such arrangement, a procedure for measuring the absolute position of the spindle 103 will be described below.
When the spindle 103 is rotated, the spindle 103 is advanced and retracted in the axial direction. At the same time, the first rotary cylinder 143 is rotated due to an engagement between the first key 144 and the first key groove 133, and the second rotary cylinder 146 is rotated due to an engagement between the second key 147 and the second key groove 134.
At this time, the first rotary cylinder 143 is rotated in synchronization with the spindle 103 since the first key groove 133 is linearly provided along the axial direction of the spindle 103. On the other hand, the second rotary cylinder 146 is rotated more slowly than the spindle 103 since the second key groove 134 is provided spirally around the axis of the spindle 103. Thus, a rotation speed difference between the two rotors 142 and 145 is generated so that the two rotary encoders 142 and 145 transmit two phase signals that are different in cycle.
Accordingly, a phase difference of the phase signals detected by the respective rotary encoders is always different, whereby a relationship as shown in FIG. 7 is observed between a rotation speed of the spindle 103 and the phase signals. For example, when a phase variation during 100 cycles is obtained from the first rotor 142 within a movable range of the spindle 103, another phase variation during 99 cycles is obtained from the second rotor 145 as shown in FIG. 7. The axial absolute position of the spindle 103 is calculated from the phase difference by taking advantage of a constant discrepancy in the phase difference between the two phase signals within the movable range of the spindle 103.
However, in such arrangement as shown in FIG. 6 of Document 1, since the two rotors 142 and 145 are provided so as to interpose the stator 141 therebetween, the two keys 144 and 147 that are provided on the rotary cylinders 143 and 146 in order to rotate the rotors 142 and 145 are spaced away from each other in the axial direction of the spindle 103 (See L1 of FIG. 6).
Since the two key grooves 133 and 134 engageable with the two keys 144 and 147 which are positioned away from each other in the axial direction of the spindle 103 must be carved on the spindle 3, start points of the respective key grooves 133 and 134 are largely shifted from each other. Consequently, a range L2 in which the key grooves 133 and 134 are carved is more widened than an axial movable range L3 of the spindle 103, which prevents downsizing of the absolute position measuring apparatus.
Further, since it is necessary that the key groove 134 is accurately machined on the outer circumference of the spindle 103, manufacturing costs are increased.